Decoupling between the diffusion coefficients that control crystal growth (DU) and viscous flow (Dη) in deeply supercooled glass-forming liquids is one of the most celebrated, still unsolved, phenomena in glass science because it strongly impacts the analysis and prediction of crystallization kinetics. To further understand the decoupling phenomenon, we measured viscosity and crystal growth rates (of the crystalline surface layer growth and internally nucleated crystals) of a supercooled liquid with barium disilicate composition. We performed all measurements using glass samples from the same batch, a rare case that we call a “clean” experiment. This type of analysis is uncommon, and its aim is to minimize the uncertainty due to the composition variation that is inherent in laboratory glass production. The decoupling temperature (Td), the glass transition temperature (Tg), the melting point (Tm), and the liquid fragility index were obtained and successfully used to test a recently derived relationship which allows one to estimate Td by knowing only Tm and the viscosity curve. We also show, for the first time, that the reduced decoupling parameter, which gauges the degree of deviation between DU and Dη, does not depend on the crystal growth mechanism for the cases of Normal or Screw Dislocation growth.
